Ametallocene is a compound typically consisting of twocyclopentadienyl anions (C
5H−
5, abbreviated Cp) bound to ametal center (M) in theoxidation state II, with the resulting general formula(C5H5)2M. Closely related to the metallocenes are the metallocene derivatives, e.g.titanocene dichloride orvanadocene dichloride. Certain metallocenes and their derivatives exhibitcatalytic properties, although metallocenes are rarely used industrially. Cationic group 4 metallocene derivatives related to [Cp2ZrCH3]+ catalyzeolefin polymerization.
Some metallocenes consist of metal plus twocyclooctatetraenide anions (C
8H2−
8, abbreviated cot2−), namely the lanthanocenes and theactinocenes (uranocene and others).
Metallocenes are a subset of a broader class of compounds calledsandwich compounds.[1]In the structure shown at right, the two pentagons are the cyclopentadienyl anions with circles inside them indicating they arearomatically stabilized. Here they are shown in astaggered conformation.
The first metallocene to be classified wasferrocene, and was discovered simultaneously in 1951 by Kealy and Pauson,[2] and Miller et al.[3] Kealy and Pauson were attempting to synthesizefulvalene through the oxidation of acyclopentadienyl salt with anhydrous FeCl3 but obtained instead the substance C10H10Fe[2] At the same time, Milleret al reported the same iron product from a reaction ofcyclopentadiene with iron in the presence of aluminum, potassium, or molybdenum oxides.[3] The structure of "C10H10Fe" was determined byGeoffrey Wilkinson et al.[1] and byErnst Otto Fischer et al.[4] These two were awarded theNobel Prize in Chemistry in 1973 for their work on sandwich compounds, including the structural determination of ferrocene.[1] They determined that the carbon atoms of the cyclopentadienyl (Cp)ligand contributed equally to the bonding and that bonding occurred due to the metald-orbitals and theπ-electrons in thep-orbitals of the Cp ligands. This complex is now known as ferrocene, and the group oftransition metal dicyclopentadienyl compounds is known as metallocenes. Metallocenes have the general formula[(η5-C5H5)2M]. Fischer et al. first prepared the ferrocene derivatives involving Co and Ni. Often derived from substituted derivatives ofcyclopentadienide, metallocenes of many elements have been prepared.[5]
One of the very earliest commercial manufacturers of metallocenes was Arapahoe Chemicals in Boulder, Colorado[6]
The general name metallocene is derived fromferrocene, (C5H5)2Fe or Cp2Fe, systematically namedbis(η5-cyclopentadienyl)iron(II). According to theInternational Union of Pure and Applied Chemistry definition, a metallocene contains atransition metal and two cyclopentadienyl ligands coordinated in a sandwich structure, i.e., the two cyclopentadienyl anions are on parallelplanes with equalbond lengths and strengths. Using the nomenclature of "hapticity", the equivalent bonding of all 5 carbon atoms of a cyclopentadienyl ring is denoted asη5, pronounced "pentahapto". There are exceptions, such asuranocene, which has twocyclooctatetraene rings sandwiching auranium atom.
In metallocene names, the prefix before the-ocene ending indicates whatmetallic element is between the Cp groups. For example, in ferrocene, iron(II), ferrous iron is present.
In contrast to the more strict definition proposed by International Union of Pure and Applied Chemistry, which requires a d-block metal and a sandwich structure, the term metallocene and thus the denotation-ocene, is applied in the chemical literature also to non-transition metal compounds, such asbarocene (Cp2Ba), or structures where the aromatic rings are not parallel, such as found inmanganocene ortitanocene dichloride (Cp2TiCl2).
Some metallocene complexes ofactinides have been reported where there are three cyclopentadienyl ligands for a monometallic complex, all three of them bound η5.[7]
There are many (η5-C5H5)–metal complexes and they can be classified by the following formulas:[8]
| Formula | Description |
|---|---|
| [(η5-C5H5)2M] | Symmetrical, classical 'sandwich' structure |
| [(η5-C5H5)2MLx] | Bent or tilted Cp rings with additional ligands, L |
| [(η5-C5H5)MLx] | Only one Cp ligand with additional ligands, L ('piano-stool' structure) |
Cp-based complexes can also be classified by type:[8]
Three main routes are normally employed in the formation of these types of compounds:[8]
Sodium cyclopentadienide (NaCp) is the preferred reagent for these types of reactions. It is most easily obtained by the reaction of molten sodium and dicyclopentadiene.[9] Traditionally, the starting point is the cracking ofdicyclopentadiene, the dimer of cyclopentadiene. Cyclopentadiene is deprotonated by strong bases or alkali metals.
NaCp acts as a reducing agent and a ligand in this reaction.
This technique provides using metal atoms in the gas phase rather than the solid metal. The highly reactive atoms or molecules are generated at a high temperature under vacuum and brought together with chosen reactants on a cold surface.
A variety of reagents have been developed that transfer Cp to metals. Once popular wasthallium cyclopentadienide. It reacts with metal halides to give thallium chloride, which is poorly soluble, and thecyclopentadienyl complex. Trialkyltin derivatives of Cp− have also been used.
Many other methods have been developed.Chromocene can be prepared fromchromium hexacarbonyl by direct reaction with cyclopentadiene in the presence ofdiethylamine; in this case, the formal deprotonation of the cyclopentadiene is followed byreduction of the resulting protons tohydrogen gas, facilitating theoxidation of the metal centre.[10]
Metallocenes generally have high thermal stability. Ferrocene can be sublimed in air at over 100 °C with no decomposition; metallocenes are generally purified in the laboratory by vacuumsublimation. Industrially, sublimation is not practical so metallocenes are isolated by crystallization or produced as part of a hydrocarbon solution. For Group IV metallocenes, donor solvents like ether or THF are distinctly undesirable for polyolefin catalysis. Charge-neutral metallocenes are soluble in common organic solvents. Alkyl substitution on the metallocene increases the solubility in hydrocarbon solvents.
A structural trend for the series MCp2 involves the variation of the M-C bonds, which elongate as the valence electron count deviates from 18.[11]
| M(C5H5)2 | rM–C (pm) | Valence electron count |
|---|---|---|
| Fe | 203.3 | 18 |
| Co | 209.6 | 19 |
| Cr | 215.1 | 16 |
| Ni | 218.5 | 20 |
| V | 226 | 15 |
In metallocenes of the type (C5R5)2M, the cyclopentadienyl rings rotate with very low barriers. Single crystalX-ray diffraction studies reveal botheclipsed orstaggered rotamers. For non-substituted metallocenes the energy difference between the staggered and eclipsed conformations is only a fewkJ/mol. Crystals of ferrocene and osmocene exhibit eclipsed conformations at low temperatures, whereas in the related bis(pentamethylcyclopentadienyl) complexes the rings usually crystallize in a staggered conformation, apparently to minimizesteric hindrance between themethyl groups.
Infrared andRaman spectroscopies have proved to be important in the analysis of cyclic polyenyl metal sandwich species, with particular use in elucidating covalent or ionic M–ring bonds and distinguishing between central and coordinated rings. Some typical spectral bands and assignments of iron group metallocenes are shown in the following table:[8]
| Ferrocene (cm−1) | Ruthenocene (cm−1) | Osmocene (cm−1) | |
|---|---|---|---|
| C–H stretch | 3085 | 3100 | 3095 |
| C–C stretch | 1411 | 1413 | 1405 |
| Ring deformation | 1108 | 1103 | 1096 |
| C–H deformation | 1002 | 1002 | 995 |
| C–H out-of-plane bend | 811 | 806 | 819 |
| Ring tilt | 492 | 528 | 428 |
| M–ring stretch | 478 | 446 | 353 |
| M–ring bend | 170 | 185 | – |
Nuclear magnetic resonance (NMR) is the most applied tool in the study of metal sandwich compounds and organometallic species, giving information on nuclear structures in solution, as liquids, gases, and in the solid state.1H NMR chemical shifts for paramagnetic organotransition-metal compounds is usually observed between 25 and 40 ppm, but this range is much more narrow for diamagnetic metallocene complexes, with chemical shifts usually observed between 3 and 7 ppm.[8]
Mass spectrometry of metallocene complexes has been very well studied and the effect of the metal on the fragmentation of the organic moiety has received considerable attention and the identification of metal-containing fragments is often facilitated by theisotope distribution of the metal. The three major fragments observed in mass spectrometry are the molecular ion peak, [C10H10M]+, and fragment ions, [C5H5M]+ and M+.[8]
After the discovery of ferrocene, the synthesis and characterization of derivatives of metallocene and other sandwich compounds attracted researchers’ interests.
Metallocenophanes feature linking of the cyclopentadienyl or polyarenyl rings by the introduction of one or more heteroannular bridges. Some of these compounds undergo thermalring-opening polymerizations to give soluble high molecular weight polymers with transition metals in the polymer backbone.Ansa-metallocenes are derivatives of metallocenes with an intramolecularbridge between the two cyclopentadienyl rings.
Triple-decker complexes are composed of three Cp anions and two metal cations in alternating order. The first triple-decker sandwich complex,[Ni2Cp3]+, was reported in 1972.[12] Many examples have been reported subsequently, often withboron-containing rings.[13]
The most famous example isferrocenium,[Fe(C5H5)2]+, the blue iron(III) complex derived from oxidation of orange iron(II) ferrocene. The lithocene anion, [Li(C5H5)2]–,[14] is the best-documented example of a metallocene anion; otherwise such ions are little known.
Many derivatives of early metal metallocenes are active catalysts forolefin polymerization. Unlike traditional and still dominant heterogeneousZiegler–Natta catalysts, metallocene catalysts are homogeneous.[8] Early metal metallocene derivatives, e.g.Tebbe's reagent,Petasis reagent, andSchwartz's reagent are useful in specialized organic synthetic operations.
The ferrocene/ferroceniumbiosensor has been discussed for determining the levels of glucose in a sample electrochemically through a series of connectedredox cycles.[8]
Metallocene dihalides [Cp2MX2] (M = Ti, Mo, Nb) exhibit anti-tumor properties, although none have proceeded far in clinical trials.[15]
